She estimated they could house 55,000 people, equivalent to around half the number of Londoners living in temporary accommodation.

Ms Jowell’s plans would see mean local authorities could levy unlimited penalties on owners of homes left vacant for six months or more.

In her article, the former Cabinet minister wrote: “Today in London, hundreds of thousands of people are stuck in temporary accommodation, on social housing waiting lists, or years of saving short of buying their first home. At the same time the global super rich buy London homes like they are gold bars, as assets to appreciate, rather than homes in which to live.”

Tessa Jowell was just one Labour MP Boris Johnson today said did not follow the official party policy regarding the mansion tax. He was speaking at the Mayor's Questions.

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